Replace `BINARYEN_ROOT` with `BINARYEN_BIN`. NFC (#9023)

This simplifies the code and also has couple of other nice benefits.

1. It matches the corresponding `--binaryen-bin` command line flags
2. Its clear that its not actually the binaryen source root so its not
tempting to think that you can find e.g. tests in `BINARYEN_ROOT/tests`.
Basically `BINARYEN_ROOT` always really means `BINARYEN_BUILD_ROOT` but
`BINARYEN_BIN` is not ambigious in the same way.

 The only folks who will be effected by this change are binaryen
 developer who use out-of-tree builds and use this environment
 variable rather than passing `--binaryen-bin=` all over the place.